1. Field of the Invention
The present invention relates to an in-plane switching liquid display (IPS-LCD), more particularly, the present invention relates to an IPS-LCD with a compensation structure for critical dimension (CD) variation.
2. Description of the Related Art
Liquid crystal displays (LCDs) may be classified by the orientation of the liquid crystal molecules between the spaced glass substrates. In a conventional twisted nematic LCD (TN-LCD), the liquid crystal molecules are twisted between the two substrates. In contrast, in an in-plane switching LCD (IPS-LCD), common electrodes and pixel electrodes are formed on a lower glass substrate (TFT substrate) and an in-plane electrode field therebetween is generated to rearrange the liquid crystal molecules along the electrode field. Accordingly, the IPS-LCD has been used or suggested for improving drawbacks of the conventional TN-LCD, such as a very narrow viewing angle and a low contrast ratio.
FIGS. 1A and 1B are sectional diagrams of a conventional IPS-LCD, in which FIG. 1A shows the alignment of the liquid crystal molecules in an off state and FIG. 1B shows the alignment of the liquid crystal molecules in an on state. The IPS-LCD has a lower glass substrate 10, an upper glass substrate 12, and a liquid crystal layer 14 disposed in a spacing between the two parallel glass substrates 10 and 12. On the lower glass substrate 10, serving as a TFT substrate, a plurality of strip-shaped common electrodes 16 is patterned on the lower glass substrate 10, an insulating layer 18 is deposited on the common electrodes 16 and the lower glass substrate 10, and a plurality of strip-shaped pixel electrodes 20 is patterned on the insulating layer 18.
As shown in FIG. 1A, before an external voltage is applied to the IPS-LCD, the negative liquid crystal molecules 14A are aligned in a direction parallel to the lower glass substrate 10. As shown in FIG. 1B, when an external voltage is applied to the IPS-LCD, an in-plane electric field is generated between the common electrode 16 and the pixel electrode 20, resulting in a rotation of the liquid crystal molecules 14B toward the in-plane electric field.
Generally, the common electrode 16 and the pixel electrode 20 are formed on the same or different layers and arranged apart from each other by a predetermined distance, known as “spacing”. For example, FIG. 2 shows a cross-section of a glass substrate having common electrodes and pixel electrodes thereon. The common electrodes 16 and the pixel electrodes 20 have a width of about 4.0 μm. The common electrodes 16 in the edge have a width of about 8.0 μm. Each spacing between a respective common electrode 16 and a respective pixel electrode 20 is about 9.0 μm in the same pixel and the adjacent pixel.
However, critical dimension (CD) variation is easily generated during formation of the common electrodes 16 and the pixel electrodes 20 caused by many parameters such as different substrate flatness, different resist thickness, and different etching recipe.
FIG. 3 is a top view showing muras on an IPS-LCD panel caused by CD variation at area B. The IPS-LCD panel 100 having area A and area B is disposed in an outer frame 102. A plurality of muras 104, curved spots, are generated on the panel 100 caused by localized CD variation.
Next, FIG. 4 shows a more detailed diagram to explain muras caused by CD variation and shows a pixel array including area A and area B having CD variation according to the prior art.
As shown in area A of FIG. 4, the pixel array comprises a plurality of small rectangles having the same numeral (10.00). Each small rectangle denotes one unit pixel that has parallel pixel electrodes 20 and parallel common electrodes 16 positioned such that a respective pixel electrode 20 is disposed adjacent and parallel to a respective common electrode 16. The numeral (10.00) in one small rectangle represents the spacing between any adjacent common electrode 16 and pixel electrode 20. The spacing between any adjacent common electrode 16 and pixel electrode 20 in the same pixel is equal to that of the adjacent pixel. For example, the spacing between any adjacent common electrode 16 and pixel electrode 20 is 10.00 μm in the pixel 30.
Turning now to area B of FIG. 4, area B shows a pixel array, having spacing CD variation of about 0.30 μm. The pixel array comprises a plurality of small rectangles having numeral (10.30) respectively. Each small rectangle denotes one unit pixel that has parallel pixel electrodes 22 and parallel common electrodes 28 positioned such that a respective pixel electrode 22 is disposed adjacent and parallel to a respective common electrode 28. The numeral (10.30) in one small rectangle represents the spacing between any adjacent common electrode 28 and pixel electrode 22. The spacing between any adjacent common electrode 28 and pixel electrode 22 in the same pixel is equal to that of the adjacent pixel. For example, the spacing between any adjacent common electrode 28 and pixel electrode 22 is 10.30 μm in the pixel 40.
FIG. 5 is a three-dimensional diagram showing transmittance difference between area A and area B according to the prior art. In FIG. 5, Z-axle represents transmittance (%), X-axle and Y-axle mean pixel unit of the pixel array of FIG. 4 including area A and area B.
FIG. 5 shows obvious transmittance difference between area A and area B so that an observer can perceive the apparent luminance difference.
Therefore, improved IPS-LCD panels formed on an active matrix substrate with a compensation structure for CD variation are needed.